1. Field
Embodiments of the invention relate to wireless communications networks, such as the Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) Long Term Evolution (LTE) and Evolved UTRAN (E-UTRAN).
2. Description of the Related Art
Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) refers to a communications network including base stations, or Node Bs (or enhanced Node Bs (eNBs) in LTE or E-UTRAN), and radio network controllers (RNC). UTRAN allows for connectivity between the user equipment (UE) and the core network. The RNC provides control functionalities for one or more Node Bs. The RNC and its corresponding Node Bs are called the Radio Network Subsystem (RNS).
Long Term Evolution (LTE) or E-UTRAN refers to improvements of the UMTS through improved efficiency and services, lower costs, and use of new spectrum opportunities. In particular, LTE is a 3GPP standard that provides for uplink peak rates of at least 50 megabits per second (Mbps) and downlink peak rates of at least 100 Mbps. LTE supports scalable carrier bandwidths from 20 MHz down to 1.4 MHz and supports both Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD).
As mentioned above, LTE is also expected to improve spectral efficiency in 3G networks, allowing carriers to provide more data and voice services over a given bandwidth. Therefore, LTE is designed to fulfill future needs for high-speed data and media transport in addition to high-capacity voice support. Advantages of LTE include high throughput, low latency, FDD and TDD support in the same platform, an improved end-user experience, and a simple architecture resulting in low operating costs. Further information on the LTE features may be found in TS 36.300 v. 11.1.0 (LTE Stage 2), which is incorporated by reference in its entirety.
LTE transmissions on the physical uplink (UL) control and shared channels (PUCCH and PUSCH) are time aligned to preserve orthogonality of PUSCH demodulation reference and PUCCH signals of the different UEs in the OFDMA system. When the proper UL timing is unknown either because UE is making initial access or there has not been UL transmissions for a long time, the random access procedure is used for setting the UL timing. Then UE transmits a preamble on the random access channel (RACH), the preamble timing being aligned with the received DL timing. As a response to the preamble, UE receives a timing advance (TA) command that indicates how much earlier the UL transmissions must be started relative to the received DL signal. After the random access procedure UE may receive TA updates that correct small timing drifts due to UE movement or radio channel changes. The validity of UE's TA value is controlled with a time alignment timer (TAT). UE resets its TAT when it receives the initial or updating TA command, and if the TAT expires, UE considers its UL unsynchronized.